Reductant injecting device, exhaust gas processing device and exhaust gas processing method

ABSTRACT

A reductant injecting device, including: a honeycomb structure including: a pillar shaped honeycomb structure portion having partition wall that defines a plurality of cells each extending from a fluid inflow end face to a fluid outflow end face; and at least one pair of electrode portions arranged on a side surface of the honeycomb structure portion; an outer cylinder having an inlet side end portion and an outlet side end portion, the inlet side end portion comprising a carrier gas introduction port being configured to introduce a carrier gas, the outlet side end portion comprising an injection port being configured to inject ammonia; a urea sprayer arranged at one end of the outer cylinder; and a spray direction switcher configured to be able to switch a spray direction of the aqueous urea solution.

FIELD OF THE INVENTION

The present invention relates to a reductant injecting device, anexhaust gas processing device, and an exhaust gas processing method.

BACKGROUND OF THE INVENTION

An exhaust gas processing device using a selective catalytic reductionNOx catalyst (SCR catalyst) is known for purifying nitrogen oxides (NOx)in exhaust gases discharged from various engines (Patent Literature 1).

The exhaust gas processing device described in Patent Literature 1includes a catalyst (SCR catalyst) attached to an exhaust gas pipe of anengine and a means for injecting urea water into the exhaust gas pipebetween the engine and the catalyst, and also includes a plurality ofurea water injection means for mixing urea water with an exhaust gas,reacting with specific components in the exhaust gas by the catalyst,and mixing the urea water with the exhaust gas.

However, in the exhaust gas processing device described in PatentLiterature 1, a temperature of the exhaust gas has to be 200° C. or morein order to decompose urea in the urea water into ammonia by the heat ofthe exhaust gas. Therefore, when the temperature of the exhaust gas islower, there is a problem that the decomposition reaction of urea isdifficult to take place, and an amount of ammonia required for the NOxtreatment is insufficient.

Therefore, an exhaust gas processing device using a reductant injectingdevice has been proposed, wherein the reductant injecting deviceincludes: a honeycomb structure (a honeycomb heater) having acylindrical honeycomb structure portion and a pair of electrode portionsarranged on a side surface of the honeycomb structure portion; and aurea sprayer being configured to spray an aqueous urea solution onto thehoneycomb structure portion (Patent Literature 2). The reductantinjecting device used in the exhaust gas processing device can spray theaqueous urea solution onto the honeycomb structure portion that has beenelectrically heated by applying a voltage to the electrode portions, anddecompose the urea in the aqueous urea solution in the honeycombstructure to produce ammonia efficiently.

However, the spraying of the aqueous urea solution onto the honeycombstructure portion electrically heated decreases a temperature of aregion where the aqueous urea solution is sprayed, thereby generating atemperature irregularity in the honeycomb structure portion. As aresult, urea deposits (crystals caused by the urea) tend to be generatedin a lower temperature region of the honeycomb structure portion. Thegeneration of the urea deposits blocks a flow path in the honeycombstructure portion, which will inhibit the decomposition of the urea intoammonia.

Therefore, exhaust gas processing devices using a reductant injectingdevice provided with a carrier gas introduction port between the ureainjector and the honeycomb structure have been proposed (PatentLiteratures 3 and 4). According to the reductant injecting device usedin each exhaust gas processing device, a carrier gas introduced from thecarrier gas introduction port can facilitate the flow of the gas in thehoneycomb structure portion. Therefore, even if the aqueous ureasolution is sprayed onto the honeycomb structure portion, a temperaturedifference in the honeycomb structure portion can be decreased, therebysuppressing the generation of urea deposits.

CITATION LIST Patent Literatures

-   [Patent Literature 1] Japanese Patent Application Publication No.    2007-327377 A-   [Patent Literature 2] WO 2014/148506-   [Patent Literature 3] Japanese Patent Application Publication No.    2017-180298 A-   [Patent Literature 4] Japanese Patent Application Publication No.    2017-180299 A

SUMMARY OF THE INVENTION

The present invention relates to a reductant injecting device,comprising:

a honeycomb structure comprising:

-   -   a pillar shaped honeycomb structure portion having partition        wall that defines a plurality of cells each extending from a        fluid inflow end face to a fluid outflow end face; and    -   at least one pair of electrode portions being configured to heat        the honeycomb structure portion by conducting a current, the        pair of the electrode portions being arranged on a side surface        of the honeycomb structure portion, the honeycomb structure        being configured to be able to decompose urea in an aqueous urea        solution in the honeycomb structure portion heated by conducting        the current to generate ammonia;

an outer cylinder having an inlet side end portion and an outlet sideend portion, the inlet side end portion comprising a carrier gasintroduction port being configured to introduce a carrier gas, theoutlet side end portion comprising an injection port being configured toinject ammonia, wherein the outer cylinder houses the honeycombstructure, and a first flow path through which a fluid can flow throughthe plurality of cells and a second flow path through which the fluidcan flow from the inlet side end portion to the outlet side end portionwithout passing through the plurality of cells are provided in the outercylinder;

a urea sprayer being configured to spray the aqueous urea solution intothe first flow path or the second flow path on an inflow side of thefluid, the urea sprayer being arranged at one end of the outer cylinder;and

a spray direction switcher configured to be able to switch a spraydirection of the aqueous urea solution to the first flow path or thesecond flow path depending on temperatures of an exhaust gas.

Further, the present invention relates to an exhaust gas processingdevice, comprising:

an exhaust gas pipe through which an exhaust gas flows;

the reductant injecting device being configured to inject ammonia or anaqueous urea solution into the exhaust gas pipe; and

an SCR catalyst arranged at the exhaust cylinder on a downstream side ofa position where ammonia or the aqueous urea solution is injected.

Furthermore, the present invention relates to a method for processing anexhaust gas, the method comprising:

injecting an aqueous urea solution into the exhaust gas from thereductant injecting device, and reducing the exhaust gas mixed with theaqueous urea solution by an SCR catalyst, when a temperature of theexhaust gas is 200° C. or higher; and

injecting generated ammonia into the exhaust gas by the reductantinjecting device, and reducing the exhaust gas mixed with the ammonia bythe SCR catalyst, when a temperature of the exhaust gas is lower than200° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a reductant injectingdevice according to Embodiment 1 of the present invention;

FIG. 2 is a schematic cross-sectional view taken along the line A-A′ inFIG. 1;

FIG. 3 is a schematic cross-sectional view showing a reductant injectingdevice according to Embodiment 2 of the present invention;

FIG. 4 is a schematic cross-sectional view taken along the line B-B′ inFIG. 3;

FIG. 5 is a schematic cross-sectional view taken along the line C-C′ inFIG. 3;

FIG. 6 is a schematic cross-sectional view showing an exhaust gasprocessing device according to Embodiment 3 of the present invention;and

FIG. 7 is a schematic cross-sectional view showing another exhaust gasprocessing device according to Embodiment 3 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An amount of NOx in an exhaust gas is changed depending on drivingpatterns of motor vehicles. When the amount of NOx in the exhaust gas ishigher, an amount of a reductant (ammonia) required for purifying NOx isincreased, so that an amount of the aqueous urea solution sprayed isneeded to be increased. The increased amount of the aqueous ureasolution sprayed decreases a temperature of the honeycomb structure, sothat an electric power for conducting a current is required to beincreased, resulting in an increased power consumption.

Further, in general, the amount of the reductant required for purifyingNOx depends on temperatures of the exhaust gas containing NOx. When thetemperature of the exhaust gas is lower, the amount of the reductantrequired is decreased, and when the temperature of the exhaust gas ishigher, the amount of reductant required is increased. Further, when thetemperature of the exhaust gas is higher (for example, 200° C. orhigher), the urea is decomposed by the heat of the exhaust gas togenerate ammonia even if the aqueous urea solution is directly sprayedinto the exhaust gas, so that NOx can be purified. Therefore, when thetemperature of the exhaust gas is lower, ammonia can be sprayed from thereductant injecting device, and when the temperature of the exhaust gasis higher, the aqueous urea solution can be sprayed from the reductantinjecting device.

However, the conventional reductant injecting device described in eachof Patent literatures 2 to 4 is not intended to spray the aqueous ureasolution as it is. Therefore, when the amount of the aqueous ureasolution sprayed is increased, the honeycomb structure portion willhinder the flow of the aqueous urea solution. This results in problemsthat the aqueous urea solution cannot be efficiently injected from thereductant injecting device, and the urea deposits are easily generated.

The present invention has been made to solve the above problems. Anobject of the present invention is to provide a reductant injectingdevice which has decreased power consumption and can select ammonia oran aqueous urea solution depending on temperatures of the exhaust gas toinject the ammonia or the aqueous urea solution efficiently.

Also, a further object of the present invention is to provide an exhaustgas processing device and an exhaust gas processing method, which canefficiently injecting ammonia or an aqueous urea solution from thereductant injecting device depending on temperatures of the exhaust gasto purify NOx.

As a result of intensive studies for a structure of a reductantinjecting device including a honeycomb structure in which at least onepair of electrode portions are arranged on a side surface of a honeycombstructure portion, the present inventors have found that the aboveproblems can be solved by forming a first flow path through which afluid can flow in cells of the honeycomb structure portion and a secondflow path through which the fluid can flow without passing through thecells of the honeycomb structure portion in an outer cylinder forhousing the honeycomb structure, and providing a spray directionswitcher that can spray an aqueous urea solution in the first flow pathor the second flow path depending on temperatures of the exhaust gas,and they have completed the present invention.

According to the present invention, it is possible to provide areductant injecting device which has decreased power consumption and canselect ammonia or an aqueous urea solution depending on temperatures ofan exhaust gas to inject the ammonia or the aqueous urea solutionefficiently.

Also, according to the present invention, it is possible to provide anexhaust gas processing device and an exhaust gas processing method,which can efficiently injecting ammonia or an aqueous urea solution fromthe reductant injecting device depending on temperatures of an exhaustgas to purify NOx.

Hereinafter, preferable embodiments of a reductant injecting device, anexhaust gas processing device and a method for processing an exhaust gasaccording to the present invention will be specifically described withreference to the drawings. It is to understand that the presentinvention is not limited to the following embodiments, and those whichappropriately added changes, improvements and the like to the followingembodiments based on knowledge of a person skilled in the art withoutdeparting from the spirit of the present invention fall within the scopeof the present invention. For example, some components may be deletedfrom all components set forth in the embodiments, or components ofdifferent embodiments may be combined as appropriate.

Embodiment 1 (1) Reductant Injecting Device

FIG. 1 is a schematic cross-sectional view showing a reductant injectingdevice according to Embodiment 1 of the present invention (a schematiccross-sectional view parallel to an extending direction of cells of ahoneycomb structure portion). Further, FIG. 2 is a schematiccross-sectional view taken along the line A-A′ in FIG. 1 (a schematiccross-sectional view perpendicular to the extending direction of thecells of the honeycomb structure portion).

As shown in FIGS. 1 and 2, a reductant injecting device 100 according tothe present embodiment includes: a honeycomb structure 1; an outercylinder 2 housing the honeycomb structure 1; and a urea sprayer 3arranged at one end of the outer cylinder 2.

The honeycomb structure 1 includes: a pillar shaped honeycomb structureportion 11 having partition wall 15 that defines a plurality of cells 14extending from a fluid inflow end face 13 a to a fluid outflow end face13 b; and at least one pair of electrode portions 12 being configured toheat the honeycomb structure portion 11 by conducting a current, theelectrode portions 12 being arranged on a side surface of the honeycombstructure portion 11. The honeycomb structure portion 11 has a hollowstructure configured such that a space region is located at a center ina cross section perpendicular to the extending direction of the cells14. In the hollow honeycomb structure portion 11 having such astructure, the cells 14 form a first flow path and the space regionforms a second flow path 16.

As used herein, the “fluid inflow end face 13 a” means an end facehaving a fluid inflow port, and the “fluid outflow end face 13 b” meansan end face having a fluid outflow port. Further, from the viewpoint ofuniformly heating the honeycomb structure portion 11, the “pair ofelectrode portions 12” are preferably arranged such that one electrodeportion 12 is arranged on an opposite side of the other electrodeportion 12 across the center of the honeycomb structure portion 11, in across section orthogonal to the extending direction of the cells 14 ofthe honeycomb structure portion 11.

The electrode portion 12 is preferably formed in a band shape along theextending direction of the cells 14. One pair of electrode portions 12are preferred, but pairs of electrode portions 12 are possible in termsof increasing a heat generation efficiency of the honeycomb structureportion 11.

The outer cylinder 2 has an inlet side end portion and an outlet sideend portion, and houses the honeycomb structure 1 therein. The inletside end portion, which is one end, is provided with a carrier gasintroduction port 22 being configured to introduce a carrier gas. Theinlet side end portion is also provided with the urea sprayer 3. Theoutlet side end, which is the other end, is provided with an injectionport 21 being configured to inject ammonia.

In the interior of the outer cylinder 2, a first flow path through whicha fluid can flow in the cells 14 of the honeycomb structure portion 11and a second flow path through which the fluid can flow from the inletside end portion to the outlet side end portion without passing throughthe cells 14 are provided. That is, in the interior of the outercylinder 2, the first flow path (cells 14) and the second flow path 16are arranged in parallel in the length direction of the outer cylinder2. Since the second flow path 16 is configured such that the fluid canflow through a portion other than the cells 14 of the honeycombstructure portion 11, the fluid flows through either the first flow path(cells 14) or the second flow path 16.

The carrier gas introduction port 22 is preferably provided on the fluidinflow end surface 13 a side of the honeycomb structure portion 11, thatis, between the honeycomb structure 1 and the urea sprayer 3.Non-limiting examples of the carrier gas that can be used includeexhaust gases, intake gases, and air from other air feeding devices(compressors and the like mounted on large vehicles and the like).

The honeycomb structure 1 housed in the outer cylinder 2 is fixed (held)in the outer cylinder 2 via an insulation holding portion 23. This canallow insulation between the honeycomb structure 1 and the outercylinder 2 to be ensured. There may be a portion (space) where theinsulation holding portion 23 is not arranged between the honeycombstructure 1 and the outer cylinder 2, but the entire outer circumferenceof the honeycomb structure 1 may be covered with the insulation holdingportion 23. A material of the insulation holding portion 23 is notparticularly limited as long as it has excellent insulating properties,and for example, alumina, glass, or the like can be used.

The urea sprayer 3 is arranged at one end (inlet side end portion) ofthe outer cylinder 2 and sprays an aqueous urea solution on the fluidinflow end surface 13 a side of the honeycomb structure portion 11.Further, the urea sprayer 3 has a spray direction switcher configured tobe able to switch a spray direction of the aqueous urea solution to thefirst flow path (cells 14) or the second flow path 16 depending ontemperatures of the exhaust gas.

The reductant injecting device 100 according to the present embodiment,which has the above structure, sprays the aqueous urea solution from theurea sprayer 3 into the first flow path (cells 14) or the second flowpath 16 depending on the temperatures of the exhaust gas. Urea in theaqueous urea solution sprayed into the first flow path (cells 14) isdecomposed by the honeycomb structure portion 11 heated by conductingthe current in the first flow path (cells 14) to generate ammonia (areductant), and the ammonia is injected to the outside via the injectionport 21. On the other hand, the aqueous urea solution sprayed into thesecond flow path 16 is directly injected to the outside via theinjection port 21. In this case, the introduction of the carrier gasinto the outer cylinder 2 can create the flow of the gas in the firstflow path (cells 14) and the second flow path 16. This prevents theaqueous urea solution from being stagnated in the first flow path (cells14) and the second flow path 16, thereby suppressing urea deposits.

Hereinafter, the reductant injecting device 100 according to the presentembodiment will be described in detail for each component.

(1-1) Honeycomb Structure 1

The honeycomb structure 1 includes the honeycomb structure portion 11and the electrode portions 12.

The partition wall 15 forming the honeycomb structure portion 11 maypreferably be made of ceramics, although not particularly limitedthereto. In particular, the partition wall 15 preferably contains asilicon-silicon carbide composite material or silicon carbide as a maincomponent, and more preferably a silicon-silicon carbide compositematerial as a main component. The use of such a material can allowelectrical resistivity of the honeycomb structure portion 11 to beeasily adjusted to any value by changing a ratio of silicon carbide andsilicon.

As used herein, the “silicon-silicon carbide composite material” as themain component means a material containing silicon carbide particles asan aggregate and metallic silicon as a binding material for bindingsilicon carbide particles. In the silicon-silicon carbide compositematerial, it is preferable that a plurality of silicon carbide particlesare bonded by metallic silicon. Further, the “silicon carbide” as themain component means a material formed by sintering silicon carbideparticles. Furthermore, as used herein, the “main component” means acomponent contained in an amount of 90% by mass or more.

The honeycomb structure portion 11 preferably has an electricalresistivity of from 0.01 to 500 Ωcm, and more preferably from 0.1 to 200Ωcm, although not particularly limited thereto. The control of theelectrical resistivity to such a level can effectively heating thehoneycomb structure portion 11 by applying a voltage to at least onepair of electrode portions 12. More particularly, in order to heat thehoneycomb structure portion 11 to 160 to 600° C. using a power sourcehaving a voltage of from 12 to 200 V, the electrical resistivity ispreferably in the above range.

The electrical resistivity of the honeycomb structure portion 11 is avalue at 25° C. The electrical resistivity of the honeycomb structureportion 11 is a value measured by a four-terminal method.

The honeycomb structure portion 11 preferably has a surface area perunit volume of from 5 cm²/cm³ or more, and more preferably from 8 to 45cm²/cm³, and particularly preferably from 20 to 40 cm²/cm³. The surfacearea of 5 cm²/cm³ or more can allow a sufficient contact area with theaqueous urea solution to be ensure, thereby appropriately controlling atreatment rate of the aqueous urea solution, i.e., an amount of ammoniagenerated (a generation rate).

The surface area of the honeycomb structure portion 11 is an area of thesurfaces of the partition wall 15 of the honeycomb structure portion 11.

The partition wall 15 of the honeycomb structure portion 11 preferablyhave a thickness of from 0.06 to 1.5 mm, and more preferably from 0.10to 0.80 mm. The thickness of the partition wall 15 of 1.5 mm or less canreduce a pressure loss, thereby appropriately controlling the treatmentrate of the aqueous urea solution, i.e., the amount of ammonia generated(generation rate). The thickness of the partition wall 15 of 0.06 mm ormore can prevent the honeycomb structure portion 11 from being destroyedby a thermal shock caused by electric conduction heating.

When the shape of each cell 14 (the shape of the cross sectionorthogonal to the extending direction of the cell 14) is circular asshown in FIG. 2, the thickness of the partition wall 15 means athickness of a portion where “a distance between the cells 14 is theshortest (a portion where the thickness of the partition wall 15 islower)”.

The cells 14 preferably have a density of from 7 to 140 cells/cm², andmore preferably from 15 to 120 cells/cm². The density of the cells 14 of7 cells/cm² or more can allow a sufficient contact area with the aqueousurea solution to be ensured, thereby appropriately controlling thetreatment rate of the urea aqueous solution, i.e., the amount of ammoniagenerated (generation rate). The density of the cells 14 of 140cells/cm² or less can reduce the pressure loss, thereby appropriatelycontrolling the treatment rate of the aqueous urea solution, i.e., theamount of ammonia generated (generation rate).

The honeycomb structure portion 11 may have some cells 14 provided withplugged portions at the end portion on the fluid inflow end surface 13 aside. The material of the plugged portions is preferably the same asthat of the partition wall 15, but other materials may be used.

A shape (outer shape) of the fluid inflow end face 13 a is notparticularly limited, and it may be various shapes such as a square, arectangle, or other polygons, and an ellipse in addition to the circularshape as shown in FIG. 2. Further, the shape (outer shape) of the fluidinflow end surface 13 a is the same as that of the fluid outflow endface 13 b, and preferably as the shape (outer shape) of the crosssection orthogonal to the extending direction of the cells 14.

The size of the honeycomb structure portion 11 is such that the areas ofthe fluid inflow end surface 13 a and the fluid outflow end surface 13 bincluding the second flow path 16 (the area of the cross sectionorthogonal to the extending direction of the cells 14) are from 50 to10000 mm², respectively, and more preferably from 100 to 8000 mm²,respectively. Further, the area of the second flow path 16 in the crosssection orthogonal to the extending direction of the cells 14 ispreferably from 20 to 2000 mm².

The shape of each cell 14 in the cross section orthogonal to theextending direction of the cell 14 is preferably an ellipse, aquadrangle, a hexagon, an octagon, or a combination thereof, in additionto the circular shape as shown in FIG. 2. Such a shape can reduce thepressure loss when the exhaust gas is passed through the honeycombstructure portion 11, thereby efficiently decomposing the urea in theaqueous urea solution.

The shape of the second flow path 16 provided at the center of thehoneycomb structure portion 11 may be, for example, in addition to thecircle as shown in FIG. 2, various shapes such as a square, a rectangle,or other polygons, and ellipses, in the cross section perpendicular tothe extending direction of the cells 14, although not particularlylimited thereto.

The shape (outer shape) of the honeycomb structure portion 11 and theshape of the second flow path 16 may be the same or different, but theyare preferably the same in terms of resistance to external impact,thermal stress, and the like.

Each electrode portion 12 is formed in a band shape along the extendingdirection of the cells 14, but it may be formed in a wider widthextending in the circumferential direction of the honeycomb structureportion 11. Further, in the cross section orthogonal to the extendingdirection of the cells 14, one electrode portion 12 is arranged on theopposite side of the other electrode portion 12 with the center of thehoneycomb structure portion 11 interposed therebetween. Such aconfiguration can allow any bias of the current flowing in the honeycombstructure portion 11 to be suppressed when the voltage is appliedbetween the pair of electrode portions 12, so that the bias of heatgeneration in the honeycomb structure portion 11 can be suppressed.

Further, the application of the voltage to the electrode portions 12preferably heats the honeycomb structure portion 11 such that thetemperature of the fluid inflow end surface 13 a is 900° C. or less. Thetemperature of the honeycomb structure portion 11 can be controlled bydirectly providing a temperature measuring means on the honeycombstructure portion 11. Alternatively, it is also possible to estimate andcontrol the temperature of the honeycomb structure portion 11 from atemperature of the carrier gas, a flow rate of the carrier gas, and theamount of the aqueous urea solution sprayed. Further, if operatingconditions of the engine are mapped, the mapping may be replaced withthe measurement of the temperature of the carrier gas and the flow rateof the carrier gas.

The material of the electrode portions 12 is preferably the same as themain component of the partition wall 15 of the honeycomb structureportion 11, although not particularly limited thereto.

The electrode portions 12 preferably have an electrical resistivity offrom 0.0001 to 100 Ωcm, and more preferably from 0.001 to 50 Ωcm. Theelectrical resistivity of the electrode portions 12 in such a range canallow the pair of electrode portions 12 to effectively play the role ofelectrodes in an exhaust gas pipe through which an exhaust gas havingelevated temperature flows. The electrical resistivity of the electrodeportions 12 is preferably lower than that of the honeycomb structureportion 11.

The electrical resistivity of the electrode portions 12 is a value at400° C. The electrical resistivity of the electrode portions 12 is avalue measured by the four-terminal method.

The pair of electrode portions 12 may be provided with electrodeterminal protruding portions 17 for connecting electrical wirings 19from the outside. The material of the electrode terminal protrudingportions 17 may be conductive ceramics or a metal. Further, the materialof the electrode terminal protruding portions 17 is preferably the sameas that of the electrode portions 12. Further, it is preferable thateach electrode terminal protruding portion 17 and a connector 18 of theouter cylinder 2 are connected by the electric wiring 19.

The honeycomb structure portion 11 may be provided with a ureahydrolysis catalyst. By using the urea hydrolysis catalyst, ammonia canbe efficiently produced from urea. Examples of the urea hydrolysiscatalyst include titanium oxide and the like.

(1-2) Outer Cylinder 2

The outer cylinder 2 is preferably made of stainless steel or the like,although not particularly limited thereto.

In order fit the outer cylinder 2 to the honeycomb structure 1, theouter cylinder 2 preferably has the same type of shape as that of thehoneycomb structure portion 11 in the cross section orthogonal to theextending direction of the cells 14. As use herein, “the same type ofshape” means that when the shape of the outer cylinder 2 is square, theshape of the honeycomb structure portion 11 is also square, and when theshape of the outer cylinder 2 is rectangular, the shape of the honeycombstructure portion 11 is also rectangular. For example, when the shapesof the outer cylinder 2 and the honeycomb structure portion 11 are ofthe same type and their shapes are rectangular, it is not necessary forboth to have the same ratio of the length to the width.

(1-3) Urea Sprayer 3

The urea sprayer 3 has a spray direction switcher configured to be ableto switch the spray direction of the aqueous urea solution to the firstflow path (cells 14) or the second flow path 16 depending on thetemperatures of the exhaust gas.

Non-limiting examples of the spray direction switcher that can be usedinclude a nozzle capable of changing the spray direction of the aqueousurea solution depending on feed pressures of the aqueous urea solution.Specific examples of the spray direction switcher that can be usedinclude a nozzle capable of spraying the aqueous urea solution into thesecond flow path 16 when the feed pressure of the aqueous urea solutionis lower, and capable of spraying the aqueous urea solution into thefirst flow path (cells 14) when the feed pressure of the aqueous ureasolution is higher. In addition, when the spray direction switcher isused, a small amount of a part of the aqueous urea solution injectedfrom the urea sprayer 3 may flow in the first flow path when the aqueousurea solution is sprayed into the second flow path 16 by the spraydirection switcher.

The type of the urea sprayer 3 is not particularly limited as long as itcan spray the aqueous urea solution. It is preferably a solenoid type,an ultrasonic type, a piezoelectric actuator type, or an atomizer type.By using these, the aqueous urea solution can be easily sprayed in theform of mists. Further, among these, the use of the solenoid type, theultrasonic type, or the piezoelectric actuator type can allow theaqueous urea solution to be sprayed in the form of mists without usingair. This can eliminate necessity of heating the air used for sprayingthe aqueous urea solution, whereby an amount of energy to be heated canbe reduced. Further, since the injection volume is reduced by not usingthe air used for spraying, a speed at which the aqueous urea solution inthe form of mists passes through the honeycomb structure portion 11 canbe reduced, resulting in a prolonged reaction time required fordecomposition. The size (diameter) of each droplet of the aqueous ureasolution sprayed from the urea sprayer 3 is preferably 0.3 mm or less.If the size of the droplet is larger than 0.3 mm, it may be difficult tovaporize it when heated in the honeycomb structure portion 11.

Here, the solenoid type urea sprayer 3 is a device that sprays theaqueous urea solution by vibrating the solenoid or moving a piston backand forth by an electric field using the solenoid. Further, theultrasonic type urea sprayer 3 is a device that sprays the aqueous ureasolution in the form of mists by ultrasonic vibration. Furthermore, thepiezoelectric actuator type urea sprayer 3 is a device that sprays theaqueous urea solution in the form of mists by vibration of apiezoelectric element. Moreover, the atomizer type urea sprayer 3 is,for example, a device that sprays the solution by sucking the solutionwith a pipe and blowing off the solution sucked up to an opening at thetip of the pipe in the form of mists using air. The atomizer type ureasprayer 3 may be a device in which a plurality of small openings areformed at the tip of the nozzle and the solution is sprayed in the formof mists from the openings.

For the urea sprayer 3, the spray direction (the direction where thedroplets eject) of the aqueous urea solution is preferably directed tothe fluid inflow end surface 13 a side of the honeycomb structureportion 11, in order to facilitate the spraying of the aqueous ureasolution on the fluid inflow end surface 13 a side of the honeycombstructure portion 11.

Next, the method for producing the reductant injecting device 100according to the present embodiment will be described in detail.

(2) Method for Producing Reductant Injecting Device 100 (2-1) Productionof Honeycomb Structure 1

When the honeycomb structure 1 is made of ceramics, the method forproducing the honeycomb structure 1 is preferably as follows:

The method for producing the honeycomb structure 1 includes: aproduction step of a honeycomb formed body; a production step of ahoneycomb dried body; a production step of a honeycomb body with unfiredelectrodes, and a production step of a honeycomb structure.

(Production Step of Honeycomb Formed Body)

The step of a honeycomb formed body preferably include extruding aforming raw material to produce a honeycomb formed body. The forming rawmaterial preferably contains a ceramic raw material and an organicbinder. In addition to the ceramic raw material and the organic binder,the forming raw material may further contain a surfactant, a sinteringaid, a pore former, water, and the like. The forming raw material can beobtained by mixing these raw materials.

The ceramic raw material in the forming raw material is “ceramics” or “araw material that will form ceramics by firing”. In any case, theceramic raw material will form ceramics after firing. The ceramic rawmaterial in the forming raw material preferably contains metallicsilicon and silicon carbide particles (silicon carbide powder) as maincomponents, or silicon carbide particles (silicon carbide powder) as amain component. This can provide the resulting honeycomb structure 1with conductivity. The metallic silicon is also preferably metallicsilicon particles (metallic silicon powder). The phrase “containsmetallic silicon and silicon carbide particles as main components” meansthat the total mass of the metallic silicon and silicon carbideparticles is 90% by mass or more of the whole (ceramic raw material).Examples of components other than the main components contained in theceramic raw material include SiO₂, SrCO₃, Al₂O₃, MgCO₃, and cordierite.

When the silicon carbide is used as the main component of the ceramicraw material, the silicon carbide is sintered by firing. Further, whenthe metallic silicon and the silicon carbide particles are used as themain components of the ceramic raw material, the silicon carbideparticles as an aggregate are bonded to each other with the metallicsilicon as a binder by firing.

When the silicon carbide particles (silicon carbide powder) and themetal silicon particles (metal silicon powder) are used as the ceramicraw materials, the mass of the metal silicon particles is preferablyfrom 10 to 40% by mass, based on the total mass of the silicon carbideparticles and the metal silicon particles. The silicon carbide particlespreferably have an average particle size of from 10 to 50 μm, and morepreferably from 15 to 35 μm. The metallic silicon particles preferablyhave an average particle size of from 0.1 to 20 μm, and more preferablyfrom 1 to 10 μm. The average particle size of each of the siliconcarbide particles and the metal silicon particles is a value measured bya laser diffraction method.

Examples of the organic binder include methyl cellulose, glycerin, andhydroxypropyl methyl cellulose. As the organic binder, one type oforganic binder may be used, or a plurality of types of organic bindersmay be used. An amount of the organic binder blended is preferably from5 to 10 parts by mass, when the total mass of the ceramic raw materialsis 100 parts by mass.

As the surfactant, ethylene glycol, dextrin and the like can be used. Asthe surfactant, one type of surfactant may be used, or a plurality oftypes of surfactants may be used. An amount of the surfactant blended ispreferably from 0.1 to 2.0 parts by mass, when the total mass of theceramic raw materials is 100 parts by mass.

The sintering aid that can be used includes SiO₂, SrCO₃, Al₂O₃, MgCO₃,cordierite and the like. As the sintering aid, one type of sintering aidmay be used, or a plurality of types of sintering aids may be used. Anamount of the sintering aid blended is preferably from 0.1 to 3 parts bymass, when the total mass of the ceramic raw materials is 100 parts bymass.

The pore former is not particularly limited as long as it forms poresafter firing. Examples include graphite, starch, foamed resins,water-absorbent resins, and silica gel. As the pore former, one type ofpore former may be used, or a plurality of types of pore formers may beused. An amount of the pore former blended is preferably from 0.5 to 10parts by mass, when the total mass of the ceramic raw materials is 100parts by mass.

An amount of water blended is preferably from 20 to 60 parts by mass,when the total mass of the ceramic raw materials is 100 parts by mass.

When the forming raw material is extruded, first, the forming rawmaterial is kneaded to form a green body. The green body is thenextruded to obtain a honeycomb formed product. The honeycomb formed bodyhas a porous partition wall 15 that defines the cells 14 each extendingfrom the fluid inflow end face 13 a to the fluid outflow end face 13 b,and is provided with the second flow path 16 at the center. Thepartition wall 15 of the honeycomb formed body is non-dried andnon-fired partition wall 15.

(Production Step of Honeycomb Dried Body)

In the step of a honeycomb dried body, first, the resulting honeycombformed body is dried to prepare a honeycomb dried body. The dryingconditions are not particularly limited, and known conditions can beused. For example, it is preferable to dry the honeycomb formed body ata temperature of from 80 to 120° C. for 0.5 to 5 hours.

(Production Step of Honeycomb Body with Unfired Electrodes)

In the production step of a honeycomb body with unfired electrodes,first, an electrode forming slurry containing the ceramic raw materialand water is applied to the side surface of the dried honeycomb body.The electrode forming slurry is then dried to form unfired electrodes toproduce a honeycomb body with unfired electrodes.

For the honeycomb body with unfired electrodes, the dried honeycomb bodyis preferably provided with wide rectangular unfired electrodes eachextending in a band shape in the extending direction of the cells 14,and also spreading in a circumferential direction. The circumferentialdirection refers to a direction along the side surface of the driedhoneycomb body in the cross section orthogonal to the extendingdirection of the cells 14.

The electrode forming slurry used in the production step of thehoneycomb body with unfired electrodes contains a ceramic raw materialand water. The electrode forming slurry may contain a surfactant, a poreformer, water, and the like.

As the ceramic raw material, it is preferable to use the ceramic rawmaterial used when producing the honeycomb formed body. For example,when the main components of the ceramic raw material used when producingthe honeycomb formed body are the silicon carbide particles and themetallic silicon, the silicon carbide particles and the metallic siliconmay also be used as the ceramic raw materials of the electrode formingslurry.

A method of applying the electrode forming slurry to the side surface ofthe dried honeycomb body is not particularly limited. The electrodeforming slurry may be applied, for example, by using a brush or by usinga printing technique.

The electrode forming slurry preferably has a viscosity of 500 Pa·s orless, and more preferably from 10 to 200 Pa·s, at 20° C. The viscosityof the electrode forming slurry of 500 Pa·s or less can lead to easyapplication of the electrode forming slurry to the side surface of thedried honeycomb body.

After applying the electrode forming slurry to the dried honeycomb body,the electrode forming slurry can be dried to obtain unfired electrodes(the honeycomb body with unfired electrodes). The drying temperature ispreferably from 80 to 120° C. The drying time is preferably from 0.1 to5 hours.

(Production Step of Honeycomb Structure)

In the production step of a honeycomb structure, the honeycomb body withthe unfired electrodes is fired to produce the honeycomb structure 1.

The firing conditions may be appropriately determined according to thetypes of the ceramic raw material used in the production of thehoneycomb formed body and the ceramic raw material used in the electrodeforming slurry.

Further, calcination is preferably carried out after drying thehoneycomb molded body with the unfired electrodes and before the firing,in order to remove the binder and the like. The calcination ispreferably carried out in an air atmosphere at a temperature of from 400to 500° C. for 0.5 to 20 hours.

When the urea hydrolysis catalyst 40 is supported on the honeycombstructure 1, for example, the honeycomb structure 1 may be immersed in acontainer in which a slurry of the urea hydrolysis catalyst 40 isstored. By adjusting a viscosity of the slurry of the urea hydrolysiscatalyst 40, a particle size of the urea hydrolysis catalyst 40contained, and the like, the catalyst can be supported not only on thesurfaces of the partition wall 15 but also in the pores of the partitionwall 15, as well as an amount of catalyst to be supported can also beadjusted. Further, the amount of the catalyst to be supported can alsobe adjusted by sucking the slurry a plurality of times.

(2-2) Production of Reductant Injecting Device 100

The reductant injecting device 100 can be produced by inserting thehoneycomb structure 1 into the outer cylinder 2, fixing the honeycombstructure 1 into the outer cylinder 2 via the insulation holding portion23, and arranging the urea sprayer 3 at one end (inlet side end portion)of the outer cylinder 2, and then connecting each of the connectors 18of the outer cylinder 2 to each of the electrode terminal protrudingportions 17 arranged on the pair of electrode portions 12 via theelectric wiring 19.

Next, a method of using the reductant injecting device 100 of thepresent embodiment will be described in detail.

(3) Method of Using Reductant Injecting Device 100

The reductant injecting device 100 according to the present embodimentsprays the aqueous urea solution used as a raw material from the ureasprayer 3 into the first flow path (cells 14) or the second flow path 16depending on the temperatures of the exhaust gas. When the temperatureof the exhaust gas is lower, the aqueous urea solution is selectivelysprayed into the first flow path (cells 14) to decompose the urea in theaqueous urea solution in the first flow path (cells 14) to generateammonia (reductant), and the ammonia is injected to the outside from theinjection port 21. More specifically, the current is conducted throughthe honeycomb structure portion 11 to increase the temperature(heating), the aqueous urea solution is fed to the urea sprayer 3, andthe aqueous urea solution is sprayed from the urea sprayer 3 to thefirst flow path (cells 14) of the honeycomb structure portion 11. Inthis case, by introducing the carrier gas from the carrier gasintroduction port 22 to the fluid inflow end surface 13 a side of thehoneycomb structure portion 11, the flow of the aqueous urea solutioncan be facilitated to prevent the aqueous urea solution from beingstagnated in the first flow path (cells 14) of the honeycomb structureportion 11. The aqueous urea solution sprayed from the urea sprayer 3enters the first flow path (cells 14) of the honeycomb structure portion11 from the fluid inflow end face 13 a according to the flow of thecarrier gas. The urea in the aqueous urea solution fed into the firstflow path (cells 14) is decomposed by the temperature of the heatedhoneycomb structure portion 11 to generate the ammonia. On the otherhand, when the temperature of the exhaust gas is higher, the aqueousurea solution is selectively sprayed into the second flow path 16 sothat the aqueous urea solution is directly injected to the outside fromthe injection port 21. In this case, by introducing the carrier gas fromthe carrier gas introduction port 22 to the fluid inflow end surface 13a side of the honeycomb structure portion 11, the flow of the aqueousurea solution can be facilitated to prevent the aqueous urea solutionfrom being stagnated in the second flow path 16 of the honeycombstructure portion 11.

Here, the temperature of the exhaust gas is related to a required amountof the reductant. That is, when the temperature of the exhaust gas ishigher, the required amount of the reductant is higher, and when thetemperature of the exhaust gas is lower, the required amount of thereductant is lower. Therefore, for example, when the temperature of theexhaust gas is 200° C. or higher, the aqueous urea solution may besprayed from the urea sprayer 3 into the second flow path 16 andinjected to the outside as it is, and when the temperature of theexhaust gas is lower than 200° C., the aqueous urea solution may besprayed from the urea sprayer 3 into the first flow path (cells 14), andthe generated ammonia may be sprayed to the outside.

The amount of the aqueous urea solution fed is not particularly limited,and it may preferably be an amount such that an equivalent ratio of theammonia to the amount of nitrogen oxides (NOx) contained in the exhaustgas is from 1.0 to 2.0. If the equivalent ratio is less than 1.0, theamount of nitrogen oxides discharged without purification may increase.However, if the SCR catalyst is provided with a NOx storage function,there may be a period during which the equivalent ratio is less than1.0. If the equivalent ratio is more than 2.0, there is a risk that theexhaust gas is likely to be discharged with the ammonia mixed in theexhaust gas.

The aqueous urea solution is preferably an aqueous solution containingfrom 10 to 40% by mass of urea, although not particularly limitedthereto. If the urea content is less than 10% by mass, it is necessaryto spray a large amount of the aqueous urea solution in order to reduceNOx, and an amount of electric power required for conducting the currentto heat the honeycomb structure portion 11 may increase. If the ureacontent is more than 40% by mass, there is a concern that the urea willsolidify in cold regions. Preferable examples of the aqueous ureasolution include AdBlue (an aqueous solution containing 32.5% by mass ofurea; a registered trademark of Verband der Automobilindustrie (VDA)),which is widely distributed in the market.

The heating temperature of the honeycomb structure portion 11 ispreferably 160° C. or higher, and more preferably from 160 to 600° C.,and even more preferably from 250 to 400° C. The heating temperature of160° C. or higher can lead to easy and efficient decomposition of theurea. The heating temperature of 600° C. or lower can allow the ammoniato be burned out and prevent the ammonia from being not fed to theexhaust gas pipe. Further, it is preferable that the heating temperatureof the honeycomb structure portion 11 is 360° C. or higher becausesulfur compounds such as ammonium hydrogen sulfate and ammonium sulfateprecipitated on the reductant injecting device 100 can be removed.

The maximum voltage applied to the honeycomb structure portion 11 ispreferably from 12 to 200 V, and more preferably from 12 to 100 V, andeven more preferably from 12 to 48 V. The maximum voltage of 12 V ormore can allow the temperature of the honeycomb structure portion 11 tobe easily increased. The maximum voltage of 200 V or less can prevent adevice for increasing the voltage from becoming expensive.

Embodiment 2

FIG. 3 is a schematic cross-sectional view showing a reductant injectingdevice according to Embodiment 2 of the present invention (a schematiccross-sectional view parallel to an extending direction of cells of ahoneycomb structure portion). Further, FIG. 4 is a schematiccross-sectional view taken along the line B-B′ in FIG. 3 (a schematiccross-sectional view perpendicular to the extending direction of thecells of the honeycomb structure portion), and FIG. 5 is a schematiccross-sectional view take along the line C-C′ in FIG. 3 (a schematiccross-sectional view parallel to the extending direction of the cells ofthe honeycomb structure portion).

As shown in FIGS. 3 to 5, in a reductant injecting device 200 accordingthe present embodiment, the honeycomb structure 1 having the rectangularpillar shaped honeycomb structure portion 11 is housed in the outercylinder 2, and in the outer cylinder 2, the second flow path (a spaceregion 16) is provided at a position adjacent to the honeycomb structure1. Further, the reductant injecting device 200 is provided with acontrol valve 31 as the spray direction switcher between the honeycombstructure 1 and the urea sprayer 3. Other configurations are the same asthose of the reductant injecting device 100 of Embodiment 1. Therefore,descriptions of the other configurations will be omitted, and onlydifferences will be described in detail.

In the reductant injecting device 200 according to the presentembodiment, the first flow path through which the fluid can flow in thecells 14 of the honeycomb structure portion 11 and the second flow pathplaced adjacent to the first flow path, through which the fluid canflow, are provided inside the outer cylinder 2. Since the second flowpath 16 is configured such that the fluid can flow through a portionother than the cells 14 of the honeycomb structure portion 11, the fluidflows through either the first flow path (cells 14) or the second flowpath 16.

The control valve 31 used as the spray direction switcher is notparticularly limited as long as it can change the spray direction of theaqueous urea solution. FIG. 5 shows a state when the control valve 31controls the spray direction of the aqueous urea solution to the secondflow path 16. Since the control valve 31 blocks the flow of the aqueousurea solution to the first flow path (cells 14) side, the aqueous ureasolution can flow to the second flow path 16. On the other hand, if theflow of the aqueous urea solution to the second flow path 16 side isblocked by the control valve 31, the aqueous urea solution can flow tothe first flow path (cells 14).

According to the reductant injecting device 200 of the presentembodiment, which has the above structure, the aqueous urea solution canbe sprayed from the urea sprayer 3 into the first flow path (cells 14)or the second flow path 16 depending on the temperatures of the exhaustgas, so that the same effects as those of the reductant injecting device100 according to Embodiment 1 can be obtained.

Embodiment 3

FIGS. 6 and 7 are schematic cross-sectional views showing exhaust gasprocessing devices according to Embodiment 3 of the present invention.

As shown in FIGS. 6 and 7, each of the exhaust gas processing devices300 and 400 according to the present embodiment include: an exhaust gaspipe 41 through which the exhaust gas flows; the reductant injectingdevice 100 being configured to inject the ammonia or aqueous ureasolution into the exhaust gas pipe 41; and an SCR catalyst 42 arrangedat the exhaust gas pipe 41 on a downstream side of a position where theammonia or aqueous urea solution is injected.

The exhaust gas pipe 41 is a pipe through which an exhaust gas (anexhaust gas containing NOx) discharged from various engines and the likeis passed, and in which the exhaust gas and ammonia are mixed. The sizeof the exhaust gas pipe 41 is not particularly limited, and it can beappropriately determined depending on exhaust systems such as engines towhich the exhaust gas processing devices 300 and 400 according thepresent embodiment are attached. The exhaust gas pipe 41 has anon-limiting length in the gas flow direction, but it preferably has alength in which a distance between the reductant injecting device 100and the SCR catalyst 42 can be set to an appropriate distance.

A material of the exhaust gas pipe 41 is not particularly limited, but amaterial that is difficult to be corroded by the exhaust gas ispreferable. Examples of the material of the exhaust gas pipe 41 includestainless steel.

In the reductant injecting device 100, the injection port 21 is attachedto the exhaust gas pipe 41, which injects the ammonia or the aqueousurea solution into the exhaust gas pipe 41. Specifically, the reductantinjecting device 100 sprays the aqueous urea solution into the exhaustgas pipe 41 when the temperature of the exhaust gas is 200° C. orhigher, and it sprays the ammonia into the exhaust gas pipe 41 when thetemperature of the exhaust gas is less than 200° C., whereby the ammoniaor the aqueous urea solution is mixed into the exhaust gas in theexhaust gas pipe 41. In addition, the aqueous urea solution sprayed intothe exhaust gas pipe 41 is decomposed into ammonia by the heat of theexhaust gas.

As shown in FIG. 6, one reductant injecting device 100 may be attachedto the exhaust gas pipe 41. Further, as shown in FIG. 7, two reductantinjecting devices may be attached to the exhaust gas pipe 41. In thiscase, the reductant injecting device on the downstream side may be thereductant injecting device 100 or a conventional reductant injectingdevice 500 (for example, the urea sprayer 3) that sprays the reductant(urea water) without heating. In a preferred embodiment, the reductionsprayer on the downstream side is the conventional reductant injectingdevice 500 that sprays the reductant (urea water) without heating. Thisis because when the amount of NOx in the exhaust gas is lower (when thetemperature of the exhaust gas is lower), most of the NOx is purified bythe reductant injecting device 100 and the SCR catalyst 42 on theupstream side, and when the amount of NOx in the exhaust gas is higher(when the temperature of the exhaust gas is higher), the urea watersprayed from the conventional reductant injecting device 500 isdecomposed into ammonia by the temperature of the exhaust gas. Further,although not shown, three or more reductant injecting devices 100 may beattached to the exhaust gas pipe 41, as needed.

When the exhaust gas is used as the carrier gas to be introduced intothe reductant injecting device 100, as shown in FIGS. 6 and 7, theexhaust gas pipe 41 is branched upstream of the reductant injectingdevice 100, and a branched flow path 43 is connected to a carrier gasintroduction port 22. On the other hand, when a carrier gas other thanthe exhaust gas (for example, an intake gas) is used, the carrier gasintroduction port 22 is connected to a carrier gas feed source via aconnection pipe or the like.

The SCR catalyst 42 in the form of a catalyst body (the honeycombstructure on which the SCR catalyst 42 is supported) is arranged at theexhaust gas pipe 41 on the downstream of the position where the ammoniaor aqueous urea solution is injected. Therefore, as shown in FIG. 6,when one reductant injecting device 100 is attached to the exhaust gaspipe 41, the SCR catalyst 42 is arranged at the exhaust gas pipe 41 onthe downstream side of the position where the reductant injecting device100 is attached. Further, as shown in FIG. 7, when two reductantinjecting devices are attached to the exhaust gas pipe 41, the SCRcatalysts 24 are arranged at the exhaust gas pipe 41 on the downstreamside of the positions where the reductant injecting device 100 and theconventional reductant injecting device 500 are attached, respectively.

Examples of the SCR catalyst 42 include vanadium-based catalysts andzeolite-based catalysts.

When the SCR catalyst 42 is used as a catalyst body supported on thehoneycomb structure, it is preferable to contain the catalyst body in acontainer and attach the container to the exhaust gas pipe 41 on thedownstream side.

The honeycomb structure that supports the SCR catalyst 42 is notparticularly limited, and honeycomb structures known in the art can beused.

It is preferable that a filter for collecting particulate matters in theexhaust gas is arranged on the upstream side of the exhaust gas pipe 41.Examples of the filter for collecting particulate matters include aceramic DPF (diesel particulate filter) 44 having a honeycomb shape.Further, it is preferable that an oxidation catalyst 45 for removinghydrocarbons and carbon monoxide in the exhaust gas is arranged on theupstream side of the exhaust gas pipe 41. The oxidation catalyst 45 ispreferably in a state of being supported on a honeycomb structure madeof ceramics (oxidation catalyst). Preferable examples of the oxidationcatalyst 45 that can be used include precious metals such as platinum(Pt), palladium (Pd), and rhodium (Rh).

When one reductant injecting device 100 is attached to the exhaust gaspipe 41, the DPF 44 and the oxidation catalyst 45 are arranged at theexhaust gas pipe 41 on the upstream side of the position where theammonia or the aqueous urea solution is injected by the reductantinjecting device 100, as shown in FIG. 6. Further, when two reductantinjecting devices 100 are attached to the exhaust gas pipe 41, the DPF44 and the oxidation catalyst 45 are arranged at the exhaust gas pipe 44on the upstream side of the position where the ammonia or the aqueousurea solution is injected by the conventional reductant injecting device500, and on the downstream side of the SCR catalyst 42 on the downstreamside of the position where the reductant injecting device 100 isattached, as shown in FIG. 7.

It is preferable to dispose an ammonia removing catalyst (oxidationcatalyst) for removing ammonia on the downstream side of the SCRcatalyst 42. Such a configuration can prevent ammonia from beingdischarged to the outside when excess ammonia that has not been used forremoving NOx in the exhaust gas flows to the downstream side. Preferableexamples of the oxidation catalyst arranged on the downstream side ofthe SCR catalyst 42 include precious metals such as platinum (Pt),palladium (Pd), and rhodium (Rh).

In the above description, the use of the reductant injecting device 100of Embodiment 1 has been described. However, the reductant injectingdevice 200 of Embodiment may be used.

Embodiment 4

In a method for processing an exhaust gas according to Embodiment 4 ofthe present invention, when the temperature of the exhaust gas is 200°C. or higher, the aqueous urea solution is injected into the exhaust gasfrom the reductant injecting device 100, 200 of Embodiment 1 or 2, andthe exhaust gas mixed with the aqueous urea solution is reduced with theSCR catalyst, and when the temperature of the exhaust gas is lower than200° C., the generated ammonia is injected into the exhaust gas by thereductant injecting device 100, 200 of Embodiment 1 or 2, and theexhaust gas mixed with the ammonia is reduced with the SCR catalyst. Themethod for processing the exhaust gas can be easily carried out by usingthe exhaust gas processing device 300, 400 according to Embodiment 3.

When the temperature of the exhaust gas is higher (when the temperatureof the exhaust gas is 200° C. or higher), the reductant injecting device100, 200 selectively sprays the aqueous urea solution, and decompose theaqueous urea solution by the heat of the exhaust gas to generateammonia, whereby the NOx can be purified by the SCR catalyst 42.Therefore, when the temperature of the exhaust gas is higher, it is notnecessary to increase the generated amount of ammonia by the reductantinjecting device 100, 200, so that the power consumption can be reducedand the generation of urea deposits can also be suppressed. On the otherhand, when the temperature of the exhaust gas is lower (when thetemperature of the exhaust gas is lower than 200° C.), the ammonia canbe selectively sprayed to purify the NOx by the SCR catalyst 42.

Further, since the carrier gas can be fed together with the aqueous ureasolution sprayed from the urea sprayer 3, the flow of the gas can befacilitated in the first flow path (cells 14) or the second flow path 16of the honeycomb structure portion 11. As a result, it is possible toprevent urea from remaining in the first flow path or the second flowpath, so that the generation limit of urea deposits can be increased.Further, when the temperature of the exhaust gas is lower than 200° C.,the ammonia decomposed by heating can be discharged to the outside withthe carrier gas, so that the reactivity can be improved.

When the ammonia is sprayed from the reductant injecting device 100, 200(when the temperature of the exhaust gas is lower than 200° C.), atemperature and flow rate of the carrier gas and a power supplied to thehoneycomb structure 1 are preferably adjusted such that a temperature ofthe fluid inflow end face 13 a of the honeycomb structure portion 11 is150° C. or higher, and preferably 250° C. or higher. In order to performsuch temperature control, for example, the temperature of the carriergas is preferably 100° C. or higher, and the flow rate is preferably 10L/min or more. The power supplied to the honeycomb structure 1 ispreferably from 150 to 500 W.

An amount of the ammonia or the aqueous urea solution injected from thereductant injecting device 100, 200 is preferably such that theequivalent ratio of ammonia to the amount of nitrogen oxide contained inthe exhaust gas is from 1.0 to 2.0. If the equivalent ratio is less than1.0, the amount of nitrogen oxides discharged without purification mayincrease. If the equivalent ratio is more than 2.0, there is a risk thatthe exhaust gas is likely to be discharged with the ammonia mixed in theexhaust gas.

It is preferable that the sprayed amount of the aqueous urea solutionand the temperature (power supply) of the honeycomb structure portion 11are controlled by an electronic control unit. Further, the temperaturemay be calculated from a resistance value of the honeycomb structureportion 11, and the temperature of the honeycomb structure portion 11may be controlled such that the calculated temperature is a desiredtemperature.

INDUSTRIAL APPLICABILITY

The reductant injecting device, the exhaust gas processing device, andthe method for processing the exhaust gas according to the presentinvention can be suitably used for purifying NOx in the exhaust gasesdischarged from various engines and the like.

DESCRIPTION OF REFERENCE NUMERALS

-   1 honeycomb structure-   2 outer cylinder-   3 urea sprayer-   11 honeycomb structure portion-   12 electrode portion-   13 a fluid inflow end face-   13 b fluid outflow end face-   14 cell-   15 partition wall-   16 second flow path-   17 electrode terminal protruding portion-   18 connector-   19 electrical wiring-   21 injection port-   22 carrier gas introduction port-   23 insulation holding portion-   31 control valve-   41 exhaust gas pipe-   42 SCR catalyst-   43 branched flow path-   44 DPF-   45 oxidation catalyst-   100, 200 reductant injecting device-   300, 400 exhaust gas processing device-   500 conventional reductant injecting device

1. A reductant injecting device, comprising: a honeycomb structurecomprising: a pillar shaped honeycomb structure portion having apartition wall that defines a plurality of cells each extending from afluid inflow end face to a fluid outflow end face; and at least one pairof electrode portions being configured to heat the honeycomb structureportion by conducting a current, the pair of the electrode portionsbeing arranged on a side surface of the honeycomb structure portion, thehoneycomb structure being configured to be able to decompose urea in anaqueous urea solution in the honeycomb structure portion heated byconducting the current to generate ammonia; an outer cylinder having aninlet side end portion and an outlet side end portion, the inlet sideend portion comprising a carrier gas introduction port being configuredto introduce a carrier gas, the outlet side end portion comprising aninjection port being configured to inject ammonia, wherein the outercylinder houses the honeycomb structure, and a first flow path throughwhich a fluid can flow through the plurality of cells and a second flowpath through which the fluid can flow from the inlet side end portion tothe outlet side end portion without passing through the plurality ofcells are provided in the outer cylinder; a urea sprayer beingconfigured to spray the aqueous urea solution into the first flow pathor the second flow path on an inflow side of the fluid, the urea sprayerbeing arranged at one end of the outer cylinder; and a spray directionswitcher configured to be able to switch a spray direction of theaqueous urea solution to the first flow path or the second flow pathdepending on temperatures of an exhaust gas.
 2. The reductant injectingdevice according to claim 1, wherein the urea in the aqueous ureasolution sprayed into the first flow path is decomposed in the firstflow path by the honeycomb structure portion heated by conducting thecurrent to generate ammonia, and the ammonia is injected to the outside,and wherein the urea aqueous solution sprayed into the second flow pathis directly injected to the outside.
 3. The reductant injecting deviceaccording to claim 1, wherein the second flow path is configured suchthat the fluid can flow through a portion other than the plurality ofcells of the honeycomb structure portion.
 4. The reductant injectingdevice according to claim 1, wherein the honeycomb structure portion isa hollow honeycomb structure portion configured such that the secondflow path is located at a center in a cross section perpendicular to anextending direction of the cells.
 5. The reductant injecting deviceaccording to claim 1, wherein the second flow path is provided at aposition adjacent to the honeycomb structure.
 6. The reductant injectingdevice according to claim 1, wherein the urea sprayer comprises a spraydirection switcher, and wherein the spray direction switcher is a nozzlecapable of changing a spray direction of the aqueous urea solutiondepending on feed pressures of the aqueous urea solution.
 7. Thereductant injecting device according to claim 1, wherein the spraydirection switcher is provided between the honeycomb structure and theurea sprayer, and the spray direction switcher is a control valvecapable of changing the spray direction of the aqueous urea solution. 8.The reductant injecting device according to claim 1, wherein the carriergas is an exhaust gas.
 9. The reductant injecting device according toclaim 1, wherein the honeycomb structure portion has an electricalresistivity of from 0.01 to 500 Ωcm.
 10. The reductant injecting deviceaccording to claim 1, wherein the honeycomb structure portion contains asilicon-silicon carbide composite material or silicon carbide as a maincomponent.
 11. The reductant injecting device according to claim 1,wherein the honeycomb structure portion has a surface area per unitvolume of from 5 cm²/cm³ or more.
 12. An exhaust gas processing device,comprising: an exhaust gas pipe through which an exhaust gas flows; thereductant injecting device according to claim 1, being configured toinject ammonia or an aqueous urea solution into the exhaust gas pipe;and an SCR catalyst arranged at the exhaust cylinder on a downstreamside of a position where ammonia or the aqueous urea solution isinjected.
 13. A method for processing an exhaust gas, the methodcomprising: injecting an aqueous urea solution into the exhaust gas fromthe reductant injecting device according to claim 1, and reducing theexhaust gas mixed with the aqueous urea solution by an SCR catalyst,when a temperature of the exhaust gas is 200° C. or higher; andinjecting generated ammonia into the exhaust gas by the reductantinjecting device, and reducing the exhaust gas mixed with the ammonia bythe SCR catalyst, when a temperature of the exhaust gas is lower than200° C.